Brain Cloud

Kyle Gabler's home page charms me as much as his simple but fun and well-executed word association game: Human Brain Cloud. You play by typing in words you associate with other words, and he grows the network of associations on the fly. The graphs are pretty to watch, easy to explore, with great motion effects; and his page of stats on players is suprisingly extensive.

Also, I adore his cartoon art. :-)

Other viral, simple online games: the Free Rice game, Just Curious (answer a question before you can ask one), the ESP game (labelling images).